We have made recombinants between an N- and a B-tropic C-type virus derived from the Balb/c mouse. That these viruses are recombinants is based on three lines of evidence (in addition to biological evidence): 1. Antigenic analysis, 2. Analysis of virion proteins by SDS-PAGE, and 3. Analysis of RNase Tl resistant oligonucleotides of parental and recombinant viruses. It is hoped that this analysis will ultimately yield a "map" of viral genes and help to identify genes or regions of the genome that determine the following phenotypes: N-, B-tropism, XC plaque morphology, and high infectivity. In addition, parental and recombinant viruses have been tested in vivo for oncogenicity. Preliminary results suggest that the parental viruses may differ in their ability to cause leukemia, that some recombinants can induce leukemia within a short latent period and that different recombinants may differ in their oncogenicity. If such differences could be confirmed, it might be possible to identify the viral genes responsible for these differences.